WikiLeaks’ self-appointed online posse is systematically abusing anyone who it believes has slighted WikiLeaks in the last week’s growing controversy over the site’s quarter million leaked diplomatic cables. And the next attack targets may be PayPal and Twitter.

MasterCard, which cut off services to WikiLeaks Tuesday, became the latest victim of a loose group of hackers known as Operation: Payback Wednesday morning, when its website went down under a flood of junk traffic sent by the group’s volunteer army. The group claimed credit for the outage on its Twitter feed: “WE ARE GLAD TO TELL YOU THAT http://www.mastercard.com/ is DOWN AND IT’S CONFIRMED! #ddos#wikileaks Operation:Payback(is a bitch!)” The twitter feed also shows that Operation: Payback launched an attack in the last 24 hours against the website of the Swedish prosecutor seeking WikiLeaks founder Julian Assange’s extradition from the UK, though that site is currently online.

PayPal and the Swiss bank PostFinance have already felt Operation: Payback’s pain in earlier cyberattacks. And now PayPal may be back on its target list, followed by Twitter. An online poster circulating Wednesday that seems to have been released by the Payback group suggests that users target PayPal.com soon. “We will fire at anyone or anything that tries to censor WikiLeaks, including multi-Billion dollar companies such as PayPal,” the poster reads. “Next target: PayPal.com. When: In a few hours.”

“Twitter, you’re next for censoring #WikiLeaks discussion,” the statement adds. Suspicion has grown among twitter users over the last several days that the service has manually prevented WikiLeaks from becoming a trending topic on the site, though Twitter has denied the rumors.

PayPal has already come under two cyberattacks after its decision to cut off its services to WikiLeaks last Friday night. On Saturday morning, Operation: Payback group claimed credit for taking down the blog where that decision was announced, Paypalblog.com. Then on Monday, PayPal says it faced another cyberattack on PayPal.com itself, which a spokesperson says caused some slowness for half an hour but failed to take down the site.

As of this posting, MasterCard’s site remained offline. A company spokesperson wrote in a statement:

“MasterCard systems have not been compromised. At this time the issue appears to be the result of a concentrated effort to flood our corporate web site with traffic and slow access. We are working to restore normal service levels. It is important to note that our systems have not been compromised and there is no impact on our cardholders’ ability to use their cards for secure transactions globally.”